There is a trend in the consumer electronics market to increase performance of electronic devices while reducing the form factor of the electronic devices. The trend to miniaturize electronic devices depends on the ability to make and implement smaller components within the electronic devices.
Optical keypads are one type of component that has been miniaturized, to a degree. Optical keypads generally include any type of input device with illuminated buttons or input regions. As one example, many types of conventional mobile phones use optical keypads with buttons, or keys, for input of alphanumeric characters.
FIG. 1 depicts a conventional optical keypad system. The conventional optical keypad system 10 includes a processor circuit 12, a light emitting diode (LED) 14, and a keypad stack. The keypad stack includes a keypad layer 16, a light guide layer 18, and a switch circuit 20. The keypad layer 16 includes several keys 22, or buttons, that are raised portions for tactile contact by a user. The keypad layer 16 is generally opaque, except for translucent portions 24 which are in the form of letters, numbers, or other symbols. The processor circuit 12 controls the LED 14 to illuminate the light guide layer 18, which generally uses total internal reflection (TIR) to distribute the light within the light guide layer 18. The light guide layer 18 includes surface feature patterns 26 (e.g., bumps or depressions) which disrupt the TIR within the light guide layer 18 and cause light to exit the light guide layer 18 towards the translucent portions 24 of the keypad layer 16. In this way, the light guide layer 18 provides backlight illumination for the keypad layer 16. The keys 22, or buttons, of the keypad layer 16 are aligned with switching devices 28 (e.g., dome switches) of the switch circuit 20, so that depression of a key 22 activates a corresponding switching device 28. The processor circuit 12 recognizes activation of the switching device 28 and may implement corresponding functionality.
In order to maintain a relatively small size of the overall electronic device, some optical keypads use a thin light guide film (LGF) to provide backlight illumination for the keys, or buttons, on the keypad. Generally, a light guide film is a planar light guide made of polycarbonate (PC) or a similar material. The light guide film is inserted behind the keypad, in between the keypad (also referred to as a keymat) and a switch circuit (e.g., a dome-pad layer). The light guide film is illuminated (e.g., by a LED) and reflects some of the light out at specific locations of the keypad. In this way, the individual keys, or buttons, on the keypad are illuminated.
While the use of a thin light guide film for backlight illumination of the keypad facilitates a relatively small implementation of an optical keypad, the use of the keys, or buttons, on the keypad are limited to the illumination of fixed characters integrated into the keypad. Hence, at a single location on the keypad, only one key character can be illuminated because the character locations are fixed on the keypad. Additionally, when the optical segments or icons on the keypad are spaced closely together, it can be difficult to separately illuminate different segments or icons of the keypad without light leakage to other segments oricons.